The present invention relates to the retorting and gasification of hydrocarbon-containing solids, particularly the retorting of oil shale.
In view of the recent rapid increases in the price of crude oil and natural gas, researchers have renewed their efforts to find alternate sources of energy and hydrocarbons. Much research has focused on recovering hydrocarbons from hydrocarbon-containing solids such as shale, tar sand or coal by pyrolysis and upon gasification of carbonaceous materials to convert solid carbonaceous material into more readily usable gaseous and liquid hydrocarbons. Other known processes involve combustion of solid carbonaceous materials with oxygen to generate energy. Pyrolysis, gasification and combustion processes typically employ a treatment zone, e.g., a reaction vessel, in which the solid is heated or reacted. The cost of these reaction zones and accompanying apparatus plays an important, often dominant part in determining the over-all economics of the process. Typically, reaction systems used can be characterized as either fluid bed, entrained bed or moving bed.
Typical of prior art schemes using a moving bed is the well-known Lurgi process. Crushed coal is fed into the top of a moving-bed gasification zone and upflowing steam endothermically reacts with the coal. Combustion of a portion of the char with oxygen below the gasification reaction zone supplies the required endothermic heat of reaction. The coal has a long residence time in the gasification reactor of about 1 hour.
A typical entrained-bed process is the well-known Koppers-Totzek process in which coal is dried, finely pulverized and injected into a treatment zone along with steam and oxygen. The coal is rapidly partially combusted, gasified and entrained by the hot gases. Residence time of the coal in the reaction zone is only a few seconds.
Typical of fluid-bed processes is the well-known Union Carbide/Battelle coal gasification process. Crushed and dried coal is injected near the bottom of a treatment zone containing a fluidized bed of coal. Heat for the reaction is provided by hot coal-ash agglomerates which drop through the fluidized bed of coal.
The above-noted processes have many disadvantages. For example, in moving-bed processes the solids residence time is long, necessitating either a very large contacting or reaction zone or a large number of reactors. In entrained-bed processes, the residence time of the solid is short, but very large quantities of hot gases must be utilized to heat the solids rapidly. In fluid-bed processes, the solids flow rate is low compared to entrained-bed processes, because gas rates must be kept low in order to maintain the solid in the fluidized state.
The use of fluidized-bed contacting zones has long been known in the art and has been widely used commercially in the fluid catalytic cracking of hydrocarbons. When a fluid is passed at a sufficient velocity upwardly through a contacting zone containing a bed of subdivided solids, the bed expands and the particles are buoyed and supported by the drag forces caused by the fluid passing through the interstices among the particles. The superficial vertical velocity of the fluid in the contacting zone at which the fluid begins to support the solids is known as the minimum fluidization velocity, and the velocity of the fluid at which the solid becomes entrained in the fluid is known as the terminal velocity. Between the minimum fluidization velocity and the terminal velocity, or entrainment velocity, the bed of solids is in a fluidized state and it exhibits the appearance and some of the characteristics of a boiling liquid.
Fluidized beds have been previously utilized in many conventional contacting processes. Fluidized beds are particularly advantageous where intimate contact between two or more fluidized solids or between solids and gases is desired. Because of the quasi-fluid or liquid-like state of the solids, there is typically a rapid over-all circulation of all the solids throughout the entire bed with substantially complete mixing, as in a stirred-tank reaction system. This rapid circulation is particularly advantageous in conventional processes in which a uniform temperature and reaction mixture is required throughout the contacting zone. On the other hand, a uniform bed temperature and provision of a uniformly mixed bed of solids is a disadvantage when it is desired to maintain a temperature gradient in the contact zone to separate or segregate various types of solids, or to carry out chemical reactions to high conversions.
Gas fluidized beds include a dense particulate phase and a bubble phase, with bubbles forming at or near the bottom of the bed. These bubbles generally grow by coalescence as they rise through the bed. Mixing and mass transfer are enhanced when the bubbles are small and evenly distributed throughout the bed. When too many bubbles coalesce so that large bubbles are formed, a surging or pounding action results, leading to less efficient heat and mass transfer.
The problem of surging or slugging in fluidized beds is not fully understood. An article by D. Geldart, Powder Technology, 7 (1973), 285-292, discusses various characteristics of fluidized beds and indicates that the phenomenon of slugging is influenced by the density of the fluidization gas, the density of the particles and the mean particle size.
Various solutions have been proposed for controlling slugging in fluidized beds. The use of baffles and other internal structural members or obstacles has been suggested, as for example in U.S. Pat. No. 2,533,026. Internal devices, however, impede over-all, substantially complete mixing of solids, which is desired in most conventional fluidized-bed processes.
U.S. Pat. No. 2,376,564 discloses a process in which a fluidized catalyst is used to catalytically crack an upflowing gaseous hydrocarbon. This patent furthermore discloses the use of a non-fluidized, heat-transfer material such as balls or pellets.
U.S. Pat. No. 3,927,996 discloses a process in which pulverized coal is carried through a portion of a bed of fluidized char. The fluidized char is introduced into a lower portion of the gasifier and reacts with steam to produce a synthesis gas.
U.S. Pat. No. 2,557,680 discloses a fluidized-bed carbonization process including a reaction zone and a regeneration zone. The reactor may contain packing material.
U.S. Pat. No. 2,700,592 discloses a fluidized-bed process for desulfurizing sulfide ores.
U.S. Pat. No. 2,868,631 discloses a fluidized bed process for gasifying coal which employs a reactor containing packing material.
U.S. Pat. No. 3,853,498 discloses a fluidized-bed process in which sand is employed for heating municipal waste.
Shale oil is not a naturally occuring product, but is formed by the pyrolysis or distillation of organic matter, commonly called kerogen, present in certain shale-like rock. The organic material has a limited solubility in ordinary solvents, making recovery by extraction uneconomical. Upon strong heating, the organic material decomposes into a gas and liquid. Residual carbonaceous material typically remains on the retorted shale.
Retorting of oil shale and other similar hydrocarbon-containing solids is basically a simple operation, which involves heating the solid material to the proper temperature and recovering the vapors evolved. However, to provide a commercially feasible process, it is necessary to consider and properly choose one of the many possible methods of physically moving the solids through a reaction, or conversion, zone in which the retorting is to be carried out as well as the many other interrelated operating parameters. The choice of a particular method of moving the solids through the reaction zone must include a consideration of the mechanical aspects as well as the chemistry in the processes involved. Further, it is necessary to consider the many possible sources of heat that may be used for the pyrolysis or destructive distillation.
In order to provide a retorting process which is economically attractive and produces the maximum amount of high-quality shale oil, the operating parameters must be carefully controlled so that the over-all process is continuous and highly reliable. Any equipment used in the process, e.g., the equipment used to provide the conversion zone, must permit a high throughput of materials, since enormous quantities of oil shale must be processed for a relatively small recovery of shale oil.
In an effort to provide an economically commercial process, many retorting processes have been proposed, offering somewhat different combinations of the many possible operating conditions and apparatus. The cost of reaction vessels and the accompanying apparatus or means for transferring reactants and heat into or from these vessels plays an important, and frequently dominant, part in determining the over-all economics of a given process. Typically the types of vessels or reactors utilized to provide the conversion zone can be characterized as being either fluid bed, entrained bed or moving bed.
Many of the disadvantages of prior art processes are avoided or overcome by the process of the present invention, which, in one aspect, involves the unique use of a combined fluidized and entrained bed process for the retorting of hydrocarbon-containing solids such as oil shale. The process of the present invention is unique in many aspects, but particularly with regard to the high throughput of the solids per unit volume of reactor coupled with the ability to retort a wide size range of solids.